Leaf-miners

At this time of the year, when trees have their green summer foliage, some horse chestnut trees, also known as conker trees, already have rather brown leaves. Looking closer their leaves have pale brown patches called leaf-mines. Leaf-mines are created by insects, usually larvae, when they feed inside the leaf and in the process produce a mine with a specific pattern.

Horse chestnut leaf-mines are the work of a micro-moth Cameraria ohridella, a native of Greece that arrived on these shores earlier this century and since then has changed the look of horse chestnut trees. Coincidentally these trees (Aesculus hippocastanum) also hail from that part of the world. They are a familiar sight in the Colchester area; their conkers are very popular with children.

Recently, while visiting friends in London, with the assistance of Sophie (5) we picked some horse chestnut leaves and decided to explore the mines.

Sophie quickly produced a needle and by carefully lifting the upper surface of the mine we were rewarded with very nice finds.

First a couple of tiny caterpillars.

The caterpillars create a mine under the upper surface of the leaf while they feed, generating a lot of dark frass (droppings). They have five instars (stages of development between moults); during the first three they feed on the sap, afterwards they undergo a hypermetamorphosis which allows them to feed on the leaf tissue and later to spin a silky cocoon. Inside it they metamorphose into pupae. But we didn’t find any pupae, only a leaf with a ‘bit sticking out’.

According to the micro-moth expert, Colin Hart, when the adult is ready to emerge the pupa wriggles itself head first out of the cocoon and into the open air. The brighter light here causes the pupa to stop wriggling and it sits there, half in and half out of the mine, this is that ‘bit sticking out’. The adult can then emerge and expand its wings. They are very small and difficult to spot. We didn’t see any but there were quite a few leaves like the one above.

The last find was a leaf with a dead larva, close to a vacated cocoon.

Regarding the mysterious row of parallel punctures, arrowed in the picture above, probably the pupa made them with the tip of its abdomen when it was wriggling itself into a vertical position. Just a guess.

Intrigued by the lack of pupae, a couple of weeks later, I inspected some more horse chestnut leaves in Hereford and found several pupae still inside their silken cocoons.  

I also found more dead caterpillars with very dark bodies, sometimes near the still occupied cocoons. Probably they have been parasitised.

The freshly emerged moths start a new generation by laying eggs on the upper surface of the leaf. It seems that in Hereford the moths were slightly less advanced than in London, as to be expected.

In the UK this prolific micro-moth has two generations, more in warmer parts of the continent. The second generation pupae overwinter in the fallen leaves inside the cocoon. The cycle starts again in the spring sometime in June when the new leaves are fully formed.

In conclusion:

Even though some trees look very sickly, this micro-moth infestation doesn’t kill them; indeed, they have survived it for years. Moreover, it has become part of the food chain: birds such as blue tits have learned to extract the caterpillar from the mines. Watch out for mines with holes when the tits are nesting!

Note that this leaf-miner attacks preferentially the common horse chestnut, Aesculus hippocastanum. It is not interested in the Indian horse chestnut, A. indica, or the horse chestnuts from north America particularly the Ohio buckeye A. glabra; or the hybrid red horse chestnuts, Aesculus × carnea. In a way, coupled with their conkers this leaf-miner is of great help with identification of the various horse chestnut trees. Have a look at them!

Acknowledgements: I’m extremely grateful to the leaf-miner expert, Dr Willem Ellis, and the micro-lepidopterist, Colin Hart, for their guidance in a totally new field to me. Also to Sophie for being such a wonderful assistant.